1. Field of the Invention
The present invention relates generally to the field of selective mass transfer through a membrane and is useful, inter alia, for heating, ventilation, and air conditioning (HVAC), gas conditioning, desiccation, distillation, desalination, fluid separation, and purification.
2. Description of the Related Art
The acquisition of a desired concentration of a particular dipole (e.g., high-dipole) moment material from a material containing the dipole moment material is a common problem faced in many applications. For example, desalination is the acquisition of a nearly 100% concentration of a high-dipole moment material, namely liquid water, from a material, namely salt water (such as seawater), containing the liquid water. Further, desiccation is the acquisition of a nearly 0% concentration of a high-dipole moment material, namely liquid water, from a material, such as moist air, containing the liquid water.
Similarly, many other applications are concerned with the acquisition of a desired concentration one or more selected materials such as dissolved ions, oxides, and the like from a material containing the one or more selected material. For example, CO2 extraction from flue gas is the acquisition of a nearly 0% concentration of one or more selected materials, namely CO2, from a material, namely flue gas, containing CO2.
A large portion of thermal energy and electrical energy is devoted to the acquisition of desired concentrations of a particular material. Therefore, a need exists for efficient systems and processes for obtaining such concentrations. Even a small improvement in efficiency may aggregate into a large energy savings.
Prior art methods of acquiring potable water from salt water, such as brackish water, seawater, and the like, include distilling the salt water through a hydrophobic porous membrane. These membranes are typically constructed from hydrophobic materials, such as PTFE or polypropylene that have been formed into a single highly porous thin layer containing a high density of very small pores. Membranes constructed in this manner are often referred to as micro-porous membranes.
Micro-porous membranes are typically used when thermally created concentration differences across the membrane allow liquid water on a first side of the membrane to evaporate through the membrane into a colder environment that is in contact or in close proximity with the opposite side of the membrane. Membrane material surrounding the pores at the liquid interface on the first side of the membrane has a low surface energy and will not allow liquid to enter. Instead, the surface tension of the water forms a meniscus or “bridge” over the entrance to these pores. Water molecules transition from a low-entropy liquid state to a high-entropy vapor state within this meniscus. The water vapor diffuses into the bulk of the membrane and transits to the other face of the membrane, where it comes in contact with the lower temperature liquid and re-condenses. In this type of membrane, the dissolved ions in the water are left within the water meniscus covering the pores at the liquid interface.
These membranes experience several failure mechanisms during use. The liquid meniscus, where the conversion to vapor occurs, concentrates the dissolved ions. Eventually the dissolved ion concentration increases to the point where the dissolved ions precipitate. These precipitated ions form a barricade over the pores curtailing the further conversion of liquid water to vapor. It is extremely difficult to re-dissolve these precipitated ions once they form the barricade. The second failure mechanism occurs when water vapor condenses within the pores of the membrane. Once enough liquid water has condensed into the pores to form a path connecting one face of the membrane to the other, dissolved ions are free to diffuse into the membrane. These dissolved ions foul the membrane internally and are difficult, if not impossible to remove.
Existing salt-water desalination plants typically use reverse osmosis membranes. These membranes are constructed from hydrophobic polymers and have a porosity and pore size such that only water can pass through the membrane leaving behind dissolved salts and minerals contained in the salt water. Because the materials used to construct these membranes are hydrophobic, a pressure differential is required to force the water through the membrane. Therefore, the salt water is pressurized to force it through the membrane.
Unfortunately, the pressure also forces contaminants that would otherwise be too large to pass through the membrane into the pore structure reducing the efficacy of the membrane. Therefore, the membrane must be cleaned by periodic back-flushing, surface scouring, and the like to remove these contaminants. In order to maintain a desired production rate of desalinated water, a reverse osmosis plant must be constructed with at least some excess capacity to allow for membrane cleaning.
Such prior art reverse osmosis processes require a considerable amount of energy to force the water through the membrane. Further, such plants are expensive due to the complexity of the piping necessary to support the pressurized operation with the necessary membrane cleaning. The reverse osmosis process is also considered unstable because it is sensitive to type and amount of dissolved ions, organic proteins, and biota in the salt water.
Therefore, a need also exists for desalination processes that are more cost-effective, more robust, and/or less energy intensive than the reverse osmosis process.